In communication systems, e.g., radio or other wireless systems, the transmitter and the receiver frequently do not have access to a common clock source. Instead, there are likely two clocks running separately, one at the transmitter and another at the receiver. In wireless systems, base stations typically have very accurate clocks. However, due to cost considerations, wireless terminals, e.g., mobile telephones and/or other mobile communications devices, often use inexpensive clocks. In many cases, such inexpensive clocks are less accurate than the clocks used in the base stations with which the mobile stations interact.
One consequence of having different clocks at the transmitter and at the receiver is that the transmitter and receiver symbol timings often drift over time even when the transmitter and the receiver are perfectly synchronized at the beginning of a communication session. In order to ensure proper communication, it is important that symbol timing synchronization be maintained, throughout a communications session.
In a known communications synchronization system, the receiver symbol timing is made a slave to the transmitter symbol timing. In the known system a receiver continuously corrects the receiver symbol timing based on the received signal. The transmitter symbol timing is simply based on the clock used at the transmitter and does not need to be corrected. The above method works well in many point-to-point communications systems and in some multiple access systems as long as received symbols from different wireless terminals are not required to be aligned at the base station.
However, in other systems, the prior art synchronization method does not work effectively. For example, in an orthogonal frequency division multiplexed (OFDM) multiple access system, a base station receives OFDM symbols from multiple wireless terminals simultaneously. For the purpose of eliminating the interference between the wireless terminals, it is advantageous to have symbols from different wireless terminals arrive at the base station receiver synchronously. As different wireless terminals are likely to have different and time varying transmitter timing, it is not feasible to adjust the base station receiver timing as a slave to the transmitter timing of the numerous individual wireless terminals with which a base station receiver may interact. Hence, the prior art synchronization system does not work effectively in OFDM multiple access systems.